When a fluid is driven to flow at a relative speed, with respect to the fluid it encounters, that exceeds the speed of sound within the encountered fluid, one or more shock waves can develop. The driving of the fluid can occur when the fluid is pressed forward by an object or body propagating through the fluid. Alternatively, the fluid can be accelerated by a pressure gradient generated by any other means, such as in wind tunnels, propulsive units, jets, and rapid heating/expansion. When a shock wave is formed in a supersonic stream of a fluid, several undesirable effects can occur.
If, for example, the supersonic stream of fluid results from a propulsive effluent stream, such as the discharge of a jet aircraft, then pressure jump(s) due to the difference in pressure across a shock wave can reduce the efficiency of the desired momentum transfer from the vehicle to the effluent stream. Additionally, a series of shock waves within the supersonic stream can augment the acoustic signature of the supersonic stream in certain frequency ranges. This augmentation of the acoustic signature is undesirable for both environmental and detection avoidance reasons. As a further example, if solid (or liquid) particles in multi-phase supersonic flow are directed to propagate across a shock wave, such as during supersonic spray deposition, a potential problem is that particles of different sizes and/or densities are affected differently when they cross the shock wave. This can result in an undesired segregation of particles, or particle size redistribution at the shock wave depending on the shock parameters and the size and/or densities of the particles. Furthermore, when a body or vehicle is driving a fluid forward, the driving body will typically feel the strong increase in pressure across the shock wave as a drag force that impedes the forward motion of the body. Another problem associated with the increase in pressure across a shock wave is an increase in temperature. Again, if the shock is being driven by a body or vehicle, high temperatures behind the shock wave can result in undesirable heating of the vehicle materials and/or components behind the shock wave. The deleterious effect of interacting shock waves and their high temperatures and pressures can be yet stronger.
The control of shock waves by reducing the strength of the shock wave or completely eliminating the shock wave is sometimes referred to as flow control. This term is used because the fluid flow is being controlled by manipulating or affecting the shock wave(s) within the fluid. When considering vehicles/bodies, flow control also encompasses processes which reduce drag. This drag can be the overall or total drag, the reduction of which is intended to optimize the performance and efficiency of the vehicle. Alternatively, the drag reduction can be preferentially applied to generate moments or torque, which is useful in maneuvering the vehicle or maintaining certain angles of attack. Flow control can also be used to reduce heating and modify acoustic signatures such as a sonic boom, which result directly from the shock waves.
As a fluid element crosses from one side of the shock wave to the other, the fluid element experiences a sharp and theoretically discontinuous increase in pressure. The magnitude of this increase or “pressure jump” is typically larger for stronger shock waves, which is characterized by a greater difference between the pressures on either side of the shock wave along a perpendicular line across the shock wave. As used herein, the term “reducing the strength” of a shock wave involves reducing the pressure difference across the shock wave along the original direction of flow by reducing or eliminating the pressure discontinuity within the fluid flow and/or diffusing or broadening the pressure jump to create a shallower pressure gradient across the shock wave in this original direction of flow. When a shock wave has been removed or eliminated, the formerly shocked flow becomes subsonic in the original direction of fluid flow although, however, the flow may be supersonic or shocked in directions transverse (not limited to orthogonal) to the original direction of the fluid flow in the specific spatial region in question.
Reducing the strength of the shock wave, or eliminating it completely, can advantageously reduce or remove a sometimes significant portion of the drag force acting on the body due to the shock wave. This can be beneficial to such bodies because a reduction in drag force increases the range and/or speed of the body. Therefore, the reduction in drag requires less energy/fuel to propel the vehicle and/or allows for a greater payload of the vehicle or body for the same amount of fuel/propellant required without invoking any drag reduction.
Another benefit of being able to reduce the strength of or eliminate the shock wave is the ability to steer the body or vehicle. If only certain portions of the shock wave are reduced in strength at a given time, such as to one side of the body, then drag on the body can be preferentially and selectively controlled. Being able to control the drag on certain parts of the body allows the body to be steered by preferentially controlling the strength of the associated shock wave(s) as well as the resulting pressure distribution along the body.
Since the first supersonic vehicle, there have been many developments to reduce the strength of shock waves; increase shock standoff distance from the vehicle; and reduce the stagnation pressure and temperature. One of the first developments was that of the aerospike 10, as illustrated in FIG. 1. This is typically a pointed protrusion extending ahead of the nose of the vehicle 12 or other critical shock-generating surfaces. The aerospike 10 effectively increases the “sharpness” of the vehicle 12, and is based on the idea of using a mechanical structure to physically push air to seed transverse motion in the fluid, thus allowing the fluid to start moving laterally out of the way before the fluid actually encounters a larger part of the vehicle 12. Because the aerospike 10 pushes air, a shock wave 14 actually begins to develop when the ambient air encounters the tip of the aerospike 10.
Other developments, as illustrated in FIG. 2, have been the injection of fluids 16, such as streams of water, gas, and heated and/or ionized fluid, toward the shock wave 14 from the vehicle 12. These fluid extensions behave similarly to the aerospike and obtain similar effects and benefits, because the counter-flowing fluid also pushes the ambient air forward and laterally before the air reaches a larger part of the vehicle 12. More recently, there have been attempts to ionize the air ahead of a vehicle and its shock wave by using radio frequency (RF) or microwave radiation. Electromagnetic methods have the benefit that they can pass through the gas without “pushing,” or imparting any momentum, to the gas. The electromagnetic radiation can therefore pass through a shock wave without significantly affecting it.
The microwave methods involve creating a spot ahead of the shock wave using a microwave intensity high enough to heat and/or ionize the gas. One proposed method, as illustrated in FIG. 3A, is to focus a microwave beam 26 emanating from the front of a supersonic vehicle 24 to a point 28 ahead of the shock wave. Another proposed method using microwaves, as illustrated in FIG. 3B, has been to mount microwave horns 20 on the wings 22 on both sides of the vehicle fuselage 24. Each microwave horn 20 emits a microwave beam 26 that is alone too weak to ionize the gas. However, when the two beams 26 are crossed in front of the vehicle 24, the combined electric field 28 is strong enough to ionize the gas. Both of the aforementioned methods using microwaves disadvantageously must be operated continually to maintain a hot and/or ionized path of gas ahead of the vehicle and/or shock wave. Furthermore, both of these methods concentrate on heating a single spot ahead of the shock wave; and as such, much of the microwave energy is inefficiently used because of the resulting scattering.
Still another development has been the use of RF antennae 30 to generate a diffuse plasma near the body of the vehicle 12, as illustrated in FIG. 4. This diffuse plasma 32 mainly affects the viscosity in the boundary layer adjacent the vehicle 12 and heats a general area around the vehicle 12.
Electric discharges 34 have also been used to ionize the air around the vehicle 12, with a resulting heating geometry similar to that of the RF generated plasma, as illustrated in FIG. 5. In this method, an electrode 36 of one polarity is positioned at the tip of the vehicle 12, and several oppositely polarized electrodes 36 are positioned along the body of the vehicle 12 further downstream. When the discharge 34 is energized, the discharge 34 results in a diffuse heating/ionization around the vehicle body 12, between the oppositely polarized electrodes 36, which tends to modify the shock wave 14.
The problem of flow control at high speeds is becoming more important as the demands on both speed and maneuverability in flight systems are increasing. As previously discussed, one approach to flow control involves mechanical manipulation of the air stream around the vehicle behind the shock wave. However, an attempt to extend an object ahead of the shock wave typically creates a shock wave of its own.
Some methods of mechanical flow control behind the shock wave use the airframe and control surfaces to divert the flow or employ impulsive lateral thrusters. However, as the speed increases to higher Mach numbers, using control surfaces to steer the body requires increasingly greater power to offset the higher pressures encountered at these speeds. These power demands typically cannot be met by the control systems designed for subsonic flow and low supersonic Mach numbers.
The increasing demands and limitations on conventional control systems have led to the desire to develop new concepts for actuators and flow control systems. It is further desired to reduce or eliminate the need for moving parts and also to work with the high speed gas flow, instead of fighting against it. It is, therefore, desirable to develop a new family of control systems whose performance is optimized at extremely high speeds. For craft that may operate at both subsonic and supersonic speeds, these systems will complement the current methods of flow control, which are very effective at low speeds but increasingly impracticable at higher speeds. There is, therefore, a need for a device with a minimal number of moving parts, and whose effectiveness increases with increasing Mach number.
Additionally, there is a need for an improved method of modifying shock waves to reduce or eliminate the pressure discontinuity within the fluid flow. Such a modification to the shock wave can eliminate or reduce associated problems with momentum transfer efficiency, particulate transfer efficiency, and/or acoustic signature. Furthermore, the modification of the shock wave can reduce heating that results from the shock wave, thereby reducing the need for complex cooling methods, reducing cost, and further expanding the performance envelope of the vehicle associated with the shock wave.
Besides increased drag, sonic boom, and destructively high temperatures and pressures on their airframe and components, the shock waves produced by hypersonic and supersonic vehicles/missiles produce additional technical challenges. For example, deploying munitions from supersonic vehicles produces further complications, as the multiple bodies and shock waves interact with each other. The problems attendant with such complications are traditionally circumvented by reducing the vehicle's speed to subsonic before deployment. However, reducing the vehicle's speed to subsonic adds new elements of risk and negates the benefits of traveling at hypersonic/supersonic speeds. Therefore, there is a need for an improved method and delivery system capable of safely and reliably deploying objects, such as munitions, while maintaining supersonic cruise conditions. Furthermore, there is a need for a system that can be retroactively applied to existing air platforms. In the realm of subsonic and transonic flight there is also room for improvement in the areas of drag reduction and flow control.